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Abstract
The Gram-negative bacterium Pseudomonas aeruginosa is one of the most dreaded pathogens in the hospital setting, and represents a prototype of multi-drug resistant “superbug” for which effective therapeutic options are very limited. The identification and characterization of new cellular functions that are essential for P. aeruginosa viability and/or virulence could drive the development of anti-Pseudomonas compounds with novel mechanisms of action. In this study we investigated whether TolB, the periplasmic component of the Tol-Pal trans-envelope protein complex of Gram-negative bacteria, represents a potential drug target in P. aeruginosa. By combining conditional mutagenesis with the analysis of specific pathogenicity-related phenotypes, we demonstrated that TolB is essential for P. aeruginosa growth, both in laboratory and clinical strains, and that TolB-depleted P. aeruginosa cells are strongly defective in cell-envelope integrity, resistance to human serum and several antibiotics, as well as in the ability to cause infection and persist in an insect model of P. aeruginosa infection. The essentiality of TolB for P. aeruginosa growth, resistance and pathogenicity highlights the potential of TolB as a novel molecular target for anti-P. aeruginosa drug discovery.
Citation: Lo Sciuto A, Fernández-Piñar R, Bertuccini L, Iosi F, Superti F, Imperi F (2014) The Periplasmic Protein TolB as a Potential Drug Target in Pseudomonas aeruginosa. PLoS ONE 9(8): e103784. https://doi.org/10.1371/journal.pone.0103784
Editor: José A. Bengoechea, Queen's University Belfast, United Kingdom
Received: April 4, 2014; Accepted: July 5, 2014; Published: August 5, 2014
Copyright: © 2014 Lo Sciuto et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: The authors confirm that all data underlying the findings are fully available without restriction. All relevant data are included within the paper and Supporting Information files.
Funding: Regina Fernández-Piñar is supported by a Post-doc fellowship from the Fundación Alfonso Martín Escudero (Spain). The work on essential periplasmic proteins in Francesco Imperi's laboratory is supported by a grant from the Pasteur Institute-Cenci Bolognetti Foundation. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Introduction
Nowadays, microbial resistance to antibiotics is a major hindrance to the successful treatment of many bacterial infections. Since resistance to a given antibiotic inevitably builds up over time, the development of new antibacterial drugs with novel mechanisms of action represents an important strategy against antibiotic resistance. Rational development of antibacterial drugs with innovative activities involves the identification of new cellular targets, that may emerge from a better understanding of cellular pathways critical for pathogen survival and/or pathogenicity.
TolB is the periplasmic component of the Tol-Pal system, a multi-protein complex present in almost all Gram-negative bacteria which connects the cytoplasmic (or inner) membrane with the outer membrane [1]. The Tol-Pal system was discovered as the protein machinery responsible for the internalization of the group A colicins and filamentous phage DNA in the model bacterium Escherichia coli [2], [3]. However, further studies have demonstrated that this system also plays a relevant role in the maintenance of cell envelope integrity and in the cell division process in almost all Gram-negative bacteria investigated to date [1], [4]–[9].
With the exception of Erwinia chrysanthemi and Caulobacter crescentus [10], [11], the Tol-Pal system does not appear to be essential for bacterial growth in vitro, as demonstrated in several Enterobacteriaceae, Pseudomonas putida and Vibrio cholerae, although Tol-Pal defective mutants generally showed increased sensitivity to toxic compounds (e.g. antibiotics) and reduced ability to cause infection [7], [8], [12]–[14]. Proteomic analysis showed that TolB is one of the most abundant proteins in the periplasm of the human pathogen Pseudomonas aeruginosa [15]. Notably, several attempts to generate tolB mutants in this bacterium, by either site-directed [16] or large-scale random transposon mutagenesis [17]–[19] failed, suggesting that tolB could indeed be essential in P. aeruginosa.
In this study, we used a conditional mutagenesis approach coupled with the analysis of specific pathogenicity-related phenotypes to verify the essentiality of the tolB gene in P. aeruginosa, and to evaluate TolB as a potential target for the development of novel anti-P. aeruginosa drugs.
Materials and Methods
Ethics statement
Human serum was obtained from five healthy volunteers who gave their written informed consent to the study. The research project was approved by the review board of the Pasteur Institute-Cenci Bolognetti Foundation, Sapienza University of Rome (Rome, Italy).
Bacteria and growth conditions
Bacterial strains and plasmids used in this study are listed in Table 1. Bacteria were grown in Mueller-Hinton (MH) broth or M9 minimal medium with 20 mM succinate (SM9) [27], containing or not arabinose at different concentrations. When indicated, sucrose was added to the growth medium to increase osmolarity.
Construction of tolB conditional mutants
Primers and restriction enzymes used for cloning are listed in Table S1. Plasmid mini-CTX1-araCPBADtolB was generated by cloning the tolB coding sequence into pBEM9 downstream to an araC-PBAD regulatory region that was optimized for P. aeruginosa by modification of the ribosome binding site [25], followed by subcloning of the entire araCPBADtolB region into the integration-proficient vector mini-CTX1 [26]. The mini-CTX1-araCPBADtolB construct was integrated into the attB neutral site of the P. aeruginosa chromosome, and the backbone plasmid removed as described [28]. In-frame deletion of the endogenous tolB copy was obtained using the sacB-based suicide vector pDM4 as previously described [24]. All constructs were verified by DNA sequencing.
Detergent, serum and antibiotic sensitivity assays
Sensitivity to the lytic effect of SDS was assessed by determining the turbidity (OD600) of bacterial cell suspensions in saline after 5-min incubation at room temperature in the presence of increasing SDS concentrations (0–5%). Serum sensitivity was determined by incubating about 108 P. aeruginosa cells at 37°C in saline in the presence of 50% human serum (pooled from five healthy volunteers) or heat-inactivated human serum [29]. Ofloxacin sensitivity was determined by incubating about 108 P. aeruginosa colony-forming units (CFUs) at 37°C in saline in the presence or in the absence of 0.5 mg/L ofloxacin, corresponding to the minimum inhibitory concentration (MIC) for the PAO1 strain [30] (data not shown). After 3 h, ten-fold serial dilutions of each cell suspension were plated on MH agar with 0.2% arabinose to determine the percentage of survival with respect to the corresponding controls. Sensitivity to polymyxin B and colistin was assessed by a modification of a previously-described assay [31]. Briefly, about 106 P. aeruginosa CFUs were incubated at 37°C in saline containing 4, 1 or 0.25 mg/L colistin or 2, 0.5 or 0.125 mg/L polymyxin B, corresponding to 4×, 1× or 0.25× MICs for the PAO1 wild-type strain, respectively (data not shown). After 1 h, ten-fold serial dilutions of each cell suspension were plated as described above to determine percentage of survival with respect to untreated controls. Resistance to the growth inhibitory activity of several antibiotics was assessed by the Kirby-Bauer disc diffusion test in MH agar supplemented or not with 0.01 or 0.005% arabinose, using disks containing gentamicin (10 µg), streptomycin (10 µg), tetracycline (30 µg), ampicillin (10 µg), ciprofloxacin (5 µg), imipenem (10 µg), ceftazidime (30 µg), colistin (10 µg) (Becton Dickinson), or polymyxin B (300 units; Oxoid). Growth inhibition halo diameters were measured after 20 or 40 h of growth at 37°C for PAO1 or the PAO1 tolB conditional mutant, respectively.
Galleria mellonella infection and persistence assays
P. aeruginosa strains were grown in MH with 0.2% arabinose, and serial dilutions of bacterial cell suspensions in saline were injected into G. mellonella larvae as described [32]. Larvae were incubated at 30°C for one week to monitor mortality. The lethal dose 90% (LD90) was determined as described [29]. P. aeruginosa persistence in G. mellonella larvae was assessed by infecting larvae with about 106 CFUs. After 2 h of incubation at 30°C, larvae were cut with a razor blade to recover the hemolymph. Ten-fold serial dilutions of the hemolymph were plated on Pseudomonas Isolation Agar containing 0.2% arabinose to determine the percentage of viable cells with respect to the initial inoculum (infecting dose).
Electron microscopy
Scanning (SEM) and transmission electron microscopy (TEM) were performed using previously described procedures [33]. For SEM examination, bacterial cells were fixed with 2.5% glutaraldehyde in 0.1 M sodium cacodylate buffer (pH 7.4) overnight at 4°C, seeded onto polylisinated glass coverslips, left to adhere for 2 h at room temperature, and postfixed with 1% OsO4 in 0.1 M sodium cacodylate buffer for 1 h at room temperature. Samples were then dehydrated through a graded series of ethanol solutions, critical point dried and gold sputtered, and examined with a SEM Inspect F (FEI) scanning electron microscope. For TEM analysis, bacterial cells were fixed with 2.5% glutaraldehyde, 2% paraformaldehyde and 2 mM CaCl2 in 0.1 M sodium cacodylate buffer (pH 7.4) overnight at 4°C. After incubation, cells were washed in cacodylate buffer and postfixed with 1% OsO4 in 0.1 M sodium cacodylate buffer for 1 h at room temperature, treated with 1% tannic acid in 0.05 M cacodylate buffer for 30 min and rinsed in 1% sodium sulphate in 0.05 cacodylate for 10 min. Fixed specimens were washed, dehydrated through a graded series of ethanol solutions (30 to 100% ethanol, each for 20 min) and embedded in Agar 100 (Agar Scientific Ltd., U.K.) (1/3 resin for 1 h and 30 min; 1/2 resin for 3 h; 2/3 resin overnight). Ultrathin sections obtained with a MT-2B Ultramicrotome (LKB – Pharmacia) were stained for 20 min with uranyl acetate (3% in 70% ethanol) and Reynold's lead citrate, and examined with an EM 208 FEI transmission electron microscope.
Results and Discussion
In order to generate a stable and unmarked P. aeruginosa tolB conditional mutant, an arabinose-inducible copy of the tolB coding sequence was inserted, together with the araC regulatory gene, into the attB neutral site of the P. aeruginosa PAO1 chromosome. Then, in-frame deletion mutagenesis was carried out in the presence of 0.2% arabinose to remove the endogenous copy of tolB, leading to the generation of the tolB conditional mutant named PAO1 ΔtolB araC-PBADtolB (Fig. 1).
An exogenous copy of the tolB coding sequence under the control of an arabinose-dependent promoter was inserted into the attB neutral site of the P. aeruginosa chromosome, by using the integration-proficient plasmid mini-CTX1-araCPBADtolB (Table 1). After Flp-mediated removal of the mini-CTX1 backbone (not shown), the resulting strain (PAO1 araCPBADtolB) is a merodiploid for tolB. In-frame deletion of the endogenous copy of tolB was obtained using the suicide plasmid pDM4ΔtolB (Table 1). Sucrose selection was carried out in the presence of arabinose to select removal of the pDM4 backbone, followed by PCR screening to identify clones carrying the tolB in-frame deletion. One of these clones was selected and used for the following analyses. This conditional mutant was named PAO1 ΔtolB araCPBADtolB.
Growth of PAO1 ΔtolB araC-PBADtolB in MH in microtiter plates was almost completely abrogated unless arabinose was added to the growth medium (Fig. 2A), and the same was observed on MH agar plates (Fig. 2B). Moreover, growth of the PAO1 tolB conditional mutant in MH broth was proportional to the concentration of arabinose in the medium (Fig. 2C), confirming that TolB expression is tightly regulated by arabinose in the PAO1 tolB conditional mutant. Comparable results were obtained in SM9 minimal medium (data not shown). These data indicate that tolB is essential for P. aeruginosa PAO1 growth under laboratory conditions, and confirm the suitability of the strategy used to generate stable conditional mutants in P. aeruginosa.
(A) Growth curves of the wild-type strain PAO1 (filled circles) and the PAO1 tolB conditional mutant in the presence (filled diamonds) or in the absence (open diamonds) of 0.2% arabinose in MH broth at 37°C in microtiter plates at 200 rpm. Results are the mean (± SD) of three independent experiments performed in triplicate. (B) Growth of PAO1 and the PAO1 tolB conditional mutant on MH agar plates with or without 0.2% arabinose (ARA) at 16 h. (C) Growth of the PAO1 tolB conditional mutant as described in legend to panel A in the presence of increasing concentrations of arabinose (0–0.2%), measured as OD600 (left panel) or CFU/ml (right panel). The graphs are representative of at least two independent experiments giving similar results. (D) Growth curves of P. aeruginosa PA14 or (E) the clinical strain TR1 (filled circles) and their corresponding tolB conditional mutants in the presence (filled squares) or in the absence (open squares) of 0.2% arabinose in MH broth at 37°C in microtiter plates at 200 rpm. Results are the mean (± SD) of two independent experiments performed in triplicate.
We then verified whether the crucial role of TolB in P. aeruginosa growth is conserved in different genetic backgrounds. To this aim, the tolB conditional mutation was introduced in the reference strain PA14 and in the clinical strain TR1, isolated from a cystic fibrosis patient [21] (Table 1). As previously observed for PAO1, the growth of both PA14 and TR1 tolB conditional mutants was strictly dependent on the addition of arabinose to the culture medium (Figs. 2D–E), strongly suggesting that the essentiality of TolB is a conserved trait in P. aeruginosa.
In order to obtain a number of TolB-deficient cells sufficient for further analyses, a dual-refresh strategy in flask was developed, using P. aeruginosa PAO1 and its isogenic tolB conditional mutant as reference strains (Fig. 3A). Cells were grown in MH broth for 14 h in the presence of 0.2% arabinose, and then two successive refreshes were performed (starting OD600 of 0.25 and 0.03, respectively) in the presence or in the absence of arabinose. As soon as a growth defect was observed in the PAO1 tolB conditional mutant grown in the absence of arabinose (dashed box in Fig. 3A), cells were collected and tested for different phenotypes related to pathogenicity and persistence (Figs. 3B–E), as well as for cellular morphology by electron microscopy (Fig. 4).
(A) Growth of PAO1 (circles) and PAO1 tolB conditional mutant (diamonds) at 37°C in MH broth at 200 rpm in flasks after two successive subcultures in the presence (filled symbols) or in the absence (open symbols) of 0.2% arabinose. The graph is representative of several assays giving similar results. (B) Lytic effect of SDS (0–5%), measured as decrease in cell suspension turbidity (OD600), on PAO1 wild-type cells (WT, filled circles), TolB-deficient mutant cells (tolB, open diamonds) and TolB-proficient mutant cells (tolB TolB+, filled diamonds). (C) Resistance of WT, tolB and tolB TolB+ to the bactericidal activity of 50% human serum or (D) to the bactericidal antibiotic ofloxacin (0.5 mg/L), expressed as percent survival compared to untreated cells. Results in panels B–D are the mean (± SD) of four independent experiments. (E) Persistence of WT, tolB and tolB TolB+ cells in G. mellonella larvae at 2 h post-infection. Sixteen larvae per group were infected in three independent assays. ***, P<0.001 (one-way ANOVA).
SEM and TEM analysis (left and right panels, respectively) of PAO1 wild-type cells (A,B), TolB-deficient mutant cells (C,D) and TolB-proficient mutant cells (E,F), grown as described in the legend to Fig. 3A. Bars: 3 µm (left panels) or 1 µm (right panels). The inset in panel D shows an enlargement of the boxed area. Abbreviations: IM, inner membrane; OM, outer membrane.
TolB-deficient cells were almost 1,000-fold more sensitive to the detergent SDS compared to wild-type or TolB-proficient mutant cells, i.e. cells of the tolB conditional mutant grown in the presence of arabinose (Fig. 3B), suggestive of major defects in cell-envelope integrity. Accordingly, electron microscopy revealed that TolB-deficient cells form multi-septate short-cell chains, characterized by abundant generation of outer membrane blebs and release of cellular content, mainly at division sites (Fig. 4 and Figure S1), indicating that TolB deficiency in P. aeruginosa negatively affects cell elongation and outer membrane invagination during cell division, as previously observed in other Gram-negative bacteria [9], [34]. However, differently from what observed for the tolB mutant of E. chrysanthemi [10], growth of the P. aeruginosa tolB conditional mutant in the absence of arabinose could not be restored by increasing the osmolarity of the culture medium with up to 20% sucrose (Figure S2), suggesting that the growth defect of this mutant is not only related to poor cell-envelope integrity.
TolB-deficient cells also showed significant defects in resistance to the antibacterial activities of both human serum (Fig. 3C) and the bactericidal antibiotic ofloxacin (Fig. 3D), measured as percent survival compared to cells treated with heat-inactivated serum or untreated cells, respectively. To further investigate the contribution of TolB to antibiotic resistance, susceptibility to the growth-inhibitory activity of different antibiotics was investigated through the Kirby-Bauer disc diffusion assay, by comparing the inhibitory halos obtained by growing the PAO1 ΔtolB araC-PBADtolB conditional mutant in the presence of low (growth permissive) concentrations of arabinose with those obtained with the P. aeruginosa PAO1 wild type (Table 2). Cells expressing low levels of TolB showed an overall increase in sensitivity to almost all antibiotics tested, including antibiotics currently used to treat P. aeruginosa infections, such as the fluoroquinolone ciprofloxacin, the carbapenem imipenem and the cephalosporin ceftazidime [35], [36]. The only exceptions were ampicillin, to which P. aeruginosa is intrinsically insensitive due to expression of the chromosomally-encoded β-lactamase AmpC [35], [37], and polymyxins (polymyxin B and colistin), for which no significant difference in susceptibility was observed between wild-type cells and mutant cells expressing low levels of TolB (Table 2). This latter result, that was obtained by growing the tolB conditional mutant in the presence of low, but growth permissive concentrations of arabinose (Table 2), was verified by performing a killing assay on wild-type and TolB-depleted mutant cells, obtained through the dual-refresh strategy shown in Figure 3A. Differently from what observed with the Kirby-Bauer assay, TolB-depleted cells were significantly more sensitive to both antibiotics than wild-type cells in the killing assay (Fig. 5), indicating that the cell envelope defects associated with complete depletion of TolB (Fig. 4) can also affect resistance to polymyxins. The cationic antimicrobial peptides colistin and polymyxin B primarily act by interacting with and disrupting the outer membrane, and then damaging the cytoplasmic membrane [38]. Thus, it is plausible that, differently from other antibiotics that need to reach intracellular targets, the activity of polymyxins in the Kirby-Bauer assay (Table 2) is poorly influenced by the lower cell envelope integrity that is presumably associated with growth in the presence of reduced TolB levels.
Survival of PAO1 wild-type cells (filled circles) and TolB-deficient mutant cells (open diamonds), obtained as shown in Figure 3A, after 1-h treatment with 4, 1 or 0.25 mg/L of colistin (left panel), or 2, 0.5 or 0.125 mg/L of polymyxin B (right panel). Values are expressed as percent survival compared to untreated cells, and the results represent the mean (± SD) of three independent experiments. **, P<0.01 (one-way ANOVA).
Since laboratory cultures not always reflect bacterial growth and virulence during infection, we also assessed the ability of the PAO1 tolB conditional mutant to cause infection and persist in the well-established G. mellonella model. This is an easy-to-handle and cost effective infection model to study P. aeruginosa pathogenicity, and a positive correlation has been observed between virulence of several P. aeruginosa mutants in G. mellonella and mice [32]. The tolB conditional mutant was strongly impaired in pathogenicity in G. mellonella, with an LD90 about 600,000 fold higher than that of the wild type (2.5 cells/larva and 1.5×106 cells/larva for PAO1 and PAO1 ΔtolB araC-PBADtolB, respectively). In order to assess the effect of TolB depletion also on P. aeruginosa persistence in vivo during the infection, G. mellonella larvae were infected with a high infecting dose (corresponding to about 106 CFUs) and the number of viable cells in the hemolymph was determined at 2 h post-infection. As shown in Fig. 3E, TolB-deficient cells displayed markedly reduced ability to persist in G. mellonella larvae with respect to wild-type or TolB-proficient mutant cells, while no significant differences in cell viability were observed between the same cell types in saline solution (Figure S3). This result indicates that TolB is also important for resistance to the antimicrobial defences of the G. mellonella hemolymph [32].
Conclusions
In this work, we demonstrated that depletion of TolB, the periplasmic component of the Tol-Pal complex, abolishes P. aeruginosa growth in vitro, and markedly reduces persistence and pathogenicity in an animal infection model, as well as resistance to human serum and several antibiotics. This evidence leads us to propose TolB as a suitable candidate for the development of new drugs against P. aeruginosa. Since TolB is a soluble protein residing in the periplasmic space [1], [15], it should be more accessible to drugs than cytosolic targets, and drug binding to TolB could delay later extrusion by efflux pumps, which represent key components of both intrinsic and acquired resistance in P. aeruginosa [39]. Considering the high level of intrinsic antibiotic resistance in P. aeruginosa [40], [41], and the overall increase in drug susceptibility observed in TolB-depleted P. aeruginosa cells (Figs. 3D and 5; Table 2), a potential anti-TolB compound could also exhibit synergism with available antibiotics, likely revitalizing some of our current therapeutic options. It should be noted that, although not essential for growth in vitro, the Tol-Pal complex is important for antibiotic resistance and pathogenicity also in other Gram-negative pathogens (reviewed in [8]), suggesting that anti-TolB therapy could be ultimately beneficial for the treatment of different bacterial infections.
Supporting Information
Figure S1.
SEM and TEM images (left and right panels, respectively) of TolB-deficient mutant cells grown as described in the legend to Figure 3A. Bars: 1 µm (left panel) or 0.5 µm (right panel).
https://doi.org/10.1371/journal.pone.0103784.s001
(PDF)
Figure S2.
Growth curves of the wild-type strain PAO1 (circles, solid lines) and the PAO1 tolB conditional mutant (diamonds, dashed lines) in microtiter plates at 37°C in MH broth supplemented with increasing concentrations of sucrose (0–20%). The graph is representative of three independent experiments giving similar results.
https://doi.org/10.1371/journal.pone.0103784.s002
(PDF)
Figure S3.
Viability of PAO1 wild-type cells (WT), TolB-deficient mutant cells (tolB) and TolB-proficient mutant cells (tolB TolB+) after 3-h incubation in saline solution at 37°C, expressed as percent survival with respect to the number of viable cells at time 0. Results are the mean (± SD) of four independent experiments. No significant differences were detected (one-way ANOVA).
https://doi.org/10.1371/journal.pone.0103784.s003
(PDF)
Acknowledgments
We are grateful to Burkhard Tümmler for P. aeruginosa TR1 and to Paul Warrener for plasmid pBEM9. We also thank Frédéric Barras for helpful discussion.
Author Contributions
Conceived and designed the experiments: F. Imperi FS. Performed the experiments: F. Imperi ALS RF LB F. Iosi. Analyzed the data: F. Imperi ALS FB. Contributed reagents/materials/analysis tools: F. Imperi FB. Contributed to the writing of the manuscript: F. Imperi FB.
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